A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Although scatterometry is a relatively quick form of analysis of a surface, measuring only the intensity of scattered radiation is not the most precise of measurements, as it does not take into account the different behavior of radiation that is polarized in different directions. For example, if the substrate object that is being measured is in the form of a grating that is aligned with one polarization direction, radiation polarized in that direction will scatter in a very different manner from radiation polarized in the orthogonal direction. To take polarization directions into account, an ellipsometric system has been envisaged that enables certain parameters of orthogonally polarized beams to be measured.
The prior art describes an ellipsometric system that enables certain parameters of orthogonally polarized beams to be measured. In one example of a known system, illumination radiation from a source is reflected from a structure on a target portion of a substrate and on its return journey from the substrate, it is linearly polarized along one of the two eigen-polarizations of three beam splitters that are present in the sensor. A first beam splitter sends part of the illumination to an imaging branch; a second beam splitter sends part of the illumination to a focus branch and a third beam splitter is a non-polarizing beam splitter that directs part of the beam to a camera CCD. Having passed through the non-polarizing beam splitter, the polarized beam passes through a phase modulator where its ordinary and extraordinary axis have been positioned at 45° with respect to the x and y directions. Subsequently, the beam is divided into its respective x- and y-polarization orientations using a Wollaston prism and impinges on a camera CCD. The relative intensities of the polarized beams are used to determine the relative polarization orientations of the different parts of the beam. From the relative polarization orientations, the effect of the structure on the beam can be determined. From the effect the structure has on the beam, the properties of the structure itself can be determined. The phase modulator is dependent on the wavelength of the radiation beam and has to be recalibrated for different types of radiation. Furthermore, the phase modulator system works only where both s- and p-polarization directions are available in the radiation to be measured. Once elliptically polarized radiation reflects from a substrate surface, depending on the azimuthal angle, either only the p-polarized radiation will be successfully reflected, only the s-polarized radiation, or a combination of the two. The combination is at a 50:50 ratio only on azimuthal angles of 45° and 135° if the polarization directions are 0° and 90°, for instance. Because of this, the information available from other azimuthal angles of the incident radiation beam is not available or at least very limited.
U.S. Pat. No. 5,880,838 (Marx et al.) also describes the measurement of a structure on a substrate using ellipsometry, wherein the measurement system is called polarization quadrature measurement (PQM). This document describes focusing a polarized beam of light (with TE and TM fields) onto the structure. The TM and TE fields are affected differently by the diffraction off the structure. The TE field can be used as a reference to analyze the phase and amplitude changes in the TM field. The relationship between phases and amplitudes of the TE and TM fields is dependent on the structural parameters (e.g. the depth of a hole or the height of a grating bar or the pitch of a grating) of the structure. By measuring this relationship, therefore, the structural parameters may be determined. Again, however, information relating to azimuthal angle of the incident radiation beam is not available.